serial: stm32: update wakeup IRQ management

The wakeup specific IRQ management is no more needed to wake up the stm32
plaform. A relationship has been established between the EXTI and
the EVENT IRQ, just need to declare the EXTI interrupt instead of the
UART event IRQ.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210319184253.5841-5-erwan.leray@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>